The present invention is directed to multiple sprocket assemblies for bicycles.
In recent years, as the number of speeds available in bicycle transmissions has increased, the number of sprocket wheels installed on the rear-wheel sprocket assembly of such bicycles has increased to 5 or 7 sprocket wheels or more. As a result, there has been a demand for simplification of the attachment structure and facilitation of the attachment method used in such cases. For example, in the rear-wheel multiple sprocket assembly described in Japanese Utility Model Application Kokai No. 63-4893, the demand is satisfied by a structure in which a plurality of sprocket wheels formed as annular plates are fit over the outer circumferential surface of a cylindrical boss member with spacers interposed between each sprocket wheel. The mutual connection of the respective sprocket wheels and spacers is accomplished by means of screws, etc, and splines are formed between the sprocket wheels and the boss member in order to prevent relative rotation between the components.
In order to reduce the weight of any multiple sprocket assembly, it is desirable to reduce the volume of the sprocket wheels. However, in the above structure the distance from the teeth of the sprocket wheel with the largest external diameter to the boss engaging portion of the sprocket wheel is fairly large, so the sprocket wheel must have a sufficient thickness in order to avoid deformation when torque is transmitted. Thus, since the sprockets in the above type of assembly must be made thicker to avoid deformation when torque is transmitted, an increase in weight is unavoidable.
In order to solve the problems encountered in the these conventional sprocket wheel units, an improved multiple sprocket assembly is disclosed in JP 4-297390. This multiple sprocket assembly comprises a spider (sprocket support) which supports a plurality of ring-shaped sprocket wheels. A light metal such as aluminum, etc., is generally used for the spider, while various types of steel materials are used for the sprocket wheels to provide adequate strength. The spider comprises [I] a boss part and [ii] a plurality of supporting arms which extend radially outward from the outer circumferential surfaces of the boss part in directions perpendicular to the axis of the boss part. The sprocket wheels are attached to mounting surfaces formed as steps in the radial direction on one side surface of each of the supporting arms. In this construction, the ring shapes of the respective sprocket wheels are set so that the sprocket wheels have a short radial width just sufficient to allow the formation of teeth and attachment holes. Accordingly, this construction is greatly improved in terms of weight reduction. However, a drawback of a multiple sprocket assembly unit constructed in this manner is that in cases where there is little difference in the respective numbers of teeth of adjacent sprocket wheels (so that there is no great difference in the diameters of the adjacent sprocket wheels), the attachment parts of one sprocket wheel overlap with the attachment parts of the other sprocket wheel in the radial direction. As a result, the radial widths of the two sprocket wheels must be increased in order to attach both sprocket wheels to the same supporting arms, so that the merit of weight reduction again is lost.